1. Field of the Invention
This invention relates generally to image processing machines, for example copying machines or scanner/printers, and more particularly to high-end color copiers and scanner/printers with features and capabilities enabling forgery of currency, securities, negotiable instruments, etc. The invention is further related to apparatus and methods for the prevention of such forgery on copying machines and the like but is also applicable to any image processing device that scans a document and has the capability to compare a scanned image to a stored image, especially for the purposes of authentication of the document or prevention of its replication.
2. Description of the Related Art
A number of systems have been proposed for recognizing valuable documents both for verification of authenticity and for prevention of forgery. These generally involve storing an accurate representation of a genuine document, scanning a new document and comparing the scanned document to the stored representation. In vending machines and money changing machines, for example, the desire is to prevent the acceptance of counterfeit currency. Bills submitted into the machines are scanned and compared to one or more acceptable currency patterns. If the scanned pattern matches a pattern of one of the genuine bills with a predetermined degree of similarity then the machine accepts the bill and delivers the change or merchandise. One such currency identification system is disclosed in U.S. Pat. No. 4,386,432, issued to Nakamura et al. Typical of such systems, Nakamura conveys the currency note or bill at a fixed speed through a fixed scanning area and detects a characteristic of the bill. In this case it is the presence of a metallic element contained in the printing ink forming the printing pattern on the bill. Such systems typically require that the entire bill be scanned in a predetermined direction and a predetermined orientation. While such requirements may not be an important limitation in vending machines or change machines where the user must insert the bill into a fixed dimension slot in a fixed orientation, such requirements make the identification system inoperative where the scanning orientation is not fixed, e.g. on a copier, where the user can place the bill in a variety of positions on the platen glass.
Alternative systems have been proposed that do not rely as much on a specific orientation of the document on the scanning support surface. Instead these systems attempt to recognize a particular aspect of the document that uniquely identifies it as one that should not be duplicated. U.S. Pat. Nos. 5,440,409 and 5,481,378, issued to Sugano et al. detect a particular color of the background of confidential documents. For example if documents are not intended to be copied the original document is printed on a red background. The document scanning section of the copier is provided with a color detecting feature which filters out all but a particular color on the scanned document. It then compares the density of the color signal to a pre-stored color table to determine if it matches the background color of documents that are not to be copied. If a match is found copying is prevented. Such systems have limited application to documents in which a particular color can be chosen as a background color and is not applicable to situations where the particular color may appear on documents that can legitimately be copied. For example, the green on a U.S. bill may appear on any number of documents that can be legitimately copied.
U.S. Pat. Nos. 5,379,093 and 5,515,451 issued to Hashimoto et al. and Tsuji et al., respectively, seek to detect a combination of characteristics of one particular feature of the document to be copied. In these patents, first a particular color is detected. Then, the image pattern having the particular color is compared to pre-stored standard patterns to detect a certain degree of similarity. Then, the pattern is analyzed to determine if it conforms to a particular shape, i.e. a circle. These systems have particular utility in detecting one unique aspect of a document, i.e. the stamp mark on bank notes issued by the Bank of Japan.
Additional systems have been proposed which seek to address the problem of document orientation on the platen glass. U.S. Pat. No. 5,216,724 , issued to Suzuki et al., and U.S. Pat. No. 5,533,144, issued to Fan, are typical of such systems. In Suzuki, for example, the document can be placed in any orientation on the platen. The document is scanned and the scanned images are processed such that the four corners of the document are determined and then the angle of the document on the platen is calculated. Then, one particular feature of the document, e.g. the red stamp mark on a Japanese bank note, is compared to a pre-stored standard pattern which corresponds generally to the calculated angle of the document. A number of standard patterns at various orientation angles are pre-stored. Fan proposes a somewhat different system in which the orientation of the document is first calculated and then the pre-stored patterns or templates are rotated to match the angle of the scanned document. In both of these systems the orientation of the document must first be calculated, which requires the detection of edges and corners.